A fuel cell system generally has a problem that the inflow of the air to the cathodes of a fuel cell may disadvantageously lead to oxidation and resulting degradation of a catalyst. In order to address this problem, for example, one proposed technique disclosed in PTL1 closes an air inlet shutoff valve and an air outlet shutoff valve at a stoppage of the system, so as to prevent the inflow of the air into the fuel cell during the stoppage of the system.
In the fuel cell system of this proposed configuration, after the air inlet shutoff valve and the air outlet shutoff valve are closed at a stoppage of the system, the pressure at the cathodes may decrease to a negative pressure level by the reaction of remaining oxygen and hydrogen in the fuel cell. A valve configured to keep the closed position irrespective of whether the pressure at the cathodes is a positive pressure level or a negative pressure level during the stoppage of the system should thus be employed as the shutoff valve to shut off the flow of the air. The configuration disclosed in PTL1 thus employs a valve of relatively complicated structure with a diaphragm and a plurality of flow inlets and flow outlets, as the shutoff valve. The shutoff valve of such complicated structure may be, however, a factor that interferes with size reduction and cost reduction of the fuel cell system.